Ceramic materials are well known for their excellent mechanical properties and stability at high temperature and have been widely used as ideal high temperature structural materials in many fields, including in the aeronautic fields. Permanent bonding between dissimilar materials to form hermetic seals is required in many products and components. Particularly stringent requirements are found in the manufacture of weapon systems, and more specifically with missile flight systems. However, although such ceramic materials exhibit desirable properties they may also exhibit brittleness that can restrict their application in fabricating structures, especially those structures with large dimensions and complex shapes. Thus, joining of ceramic components and especially dissimilar ceramic components can present challenges. Traditional joining or bonding technologies like mechanical connection, diffusion bonding and brazing are used for ceramic-to-ceramic connections, however, each of these known techniques have drawbacks.
In situations where the two materials have dissimilar thermal expansion coefficients, temperature fluctuations may induce fractures or permanent deformation that either cause the two different materials to break apart or shift in position relative to each other. The temperature changes can reflect cooling from the processing temperature at which the parts were bonded or from temperature cycles during the lifetime of the product containing the bonded components. Attempts to avoid the challenges of bonding dissimilar ceramic components have included, trying to select materials such that the mismatch of the different coefficients of thermal expansion (CTE) are minimized, performing bonding at the lowest possible temperature to avoid residual forces that can be locked in during cooling, minimizing the bonding area, use of a compliant layer that will absorb some of the thermal mismatch, incorporation of a multi-layer bonding system where each layer provides a gradual step change in the mismatched CTE, and even product design changes in an attempt to place the bond areas in locations of minimal relative movement of the joined parts.
Although there has been limited success with known methods of bonding ceramic components, challenges still exist in bonding dissimilar ceramic components, especially those components with different CTEs. For example, the known bonding methods can place severe restrictions on the materials that can be joined, use curing temperatures that are lower than thermal cycle temperatures experienced by the produced part, form low bond strength due to minimal bond area, cause shifting of bond layers due to thermal cycles, and involve high cost of multi-layer bonding techniques and/or design changes.
Accordingly, there is a need for improved adhesive compositions and bonding methods that avoid or minimize the issues associated with known bonding techniques and provides a cost effective solution to bonding dissimilar ceramic components.